This invention relates to an improved method for the utilization of spectrometers. More particularly, this invention relates to an improved method for transferring spectral information, including chemometric models, among spectrometers.
The physical properties of sample materials, which, for purposes of the present invention encompass the physical, chemical, and fuel properties of sample materials, have historically been measured one property at a time, utilizing test methods which have been developed to specifically evaluate one particular property. For example, the heat of formation of a particular sample has been determined by actually burning the sample in a calorimeter. Similarly, the molecular weight of a sample has been determined by inducing and measuring viscous flow of the sample using a viscometer. In each of these examples, however, the physical test methods measure, or quantify, the physical properties by actually subjecting the sample to the conditions in question. To measure more than one physical property of a particular sample, a plurality of tests must be individually conducted on a plurality of samples. Often these samples are destroyed or consumed in the process. These approaches to measuring physical properties are slow, expensive, subject to testing inconsistency, and do not facilitate on-line or real time use in an industrial or field setting.
More recently, spectrophotometric analysis has been used to determine indirectly the quantitative properties of sample materials.
U.S. Pat. No. 4,800,279 to Hieftje et al. discloses a method for utilizing near-infrared absorbance spectra to identify the physical properties of gaseous, liquid, or solid samples. The method requires measuring and recording the near-infrared absorbance spectra of a representative field of calibration samples and employing a row-reduction algorithm to determine which wavelengths in the near-infrared spectrum, and associated weighting constants, are statistically correlated to the physical property being quantified. The near-infrared absorbance of a sample can then be measured at each of the correlated wavelengths and corrected by the corresponding weighting constants. The physical property being quantified is then computed from the corrected measure of the absorbance of the sample at the correlated wavelengths.
Use of spectrophotometric analysis has numerous advantages over other methods since it is rapid, relatively inexpensive, and multivariate in that many properties can be tested for simultaneously. There is theoretically great potential for spectrophotometric analysis in manufacturing facilities, chemical plants, petroleum refineries, and the like. However, several obstacles must be overcome in order to achieve successful implementation from a practical viewpoint.
Chemometric predictions are particularly sensitive to aberrations in the sample spectra and in the spectra developed from the original calibration standards. These spectra are generally presented as plots of optical wavelength, frequency of radiation, or the like (x-axis) and absorbance, transmittance, or intensity of light (y-axis). The wavelength measurements can be, and are generally made over a range of at least 100 nanometers. Shifts of 0.01 nanometers in the wavelength axis can produce measurable and often sizeable errors in predictions. When chemometric models are used directly on another seemingly identical spectrometer (i.e. produced by the same manufacturer and having the same model number). the predictions can be and are generally grossly inaccurate. Often, chemometric models have to be rebuilt after simple repairs or maintenance of a spectrometer. The model rebuilding step can require the collection and the analysis of from 20 to 100 samples prior to proceeding with chemometric model rebuilding. These activities are costly, time consuming, and subject the model to additional errors in laboratory analysis.
Several patents teach methods for calibrating single spectrometers or cross-correlating among spectrometers, each method achieving varying degrees of success and imposing limitations as to its use.
U.S. Pat. No. 4,744,657 to Aralis et al. discloses a method for calibrating a single spectrometer, suitable for use with spectrometers having a monochromatic light source. The method addresses calibration of the photometric axis (light intensity) only and avoids the necessity of addressing variances in the wavelength axis by using a light source having a series of monochromatic lines. While single-wavelength monochromatic spectrometers can be easier to calibrate, they are limited as to the wavelengths that can be monitored, resulting in the frequent switching of wavelengths and light sources and resulting in numerous calibrations. Moreover, outside of a controlled laboratory environment, optical components are subjected to external stresses which can cause drift in the baseline. This drift is generally eliminated by filtering techniques which require measurements of light intensity at consecutive wavelengths rather than at a single wavelength.
U.S. Pat. No. 4,779,216 to Collins discloses a two-stage interactive method for calibrating a single spectrometer having a polychromatic light source. The method requires use of a large number of monochromatic spectral lines (approximately 100 lines) and a small spectral window to obtain precise wavelength calibration. An iterative, self-consistent, discrete Fouder transform is used for the determination of multiple positioning correction terms. When the Fourier calculations are completed, the results of the calibration procedure are presented to a skilled analyst for acceptance. The method requires use of complex Fourier transforms, relies extensively on skilled analyst intervention to accomplish the calibration, and is generally limited to use on moving grating spectrometers or other spectrometers where the resolution can be varied.
U.S. Pat. No. 4,866,644 to Shenk et al. discloses a method for cross-correlating spectrometers by the statistical treatment of spectra for several representative samples of the material to be analyzed. The method involves measuring the spectra of the representative samples on both spectrometers and developing statistical correlations to permit the use of spectral information obtained from one instrument on the second instrument. The representative samples are generally maintained and stored in an extensive sample library for future calibrations. The use of extensive sample libraries can be inconvenient and difficult to utilize in an industrial environment, especially in facilities where several spectrometers are actively in use. Furthermore, there is no guarantee that the sample properties will not change during storage due to chemical instability and contamination.
The above U.S. Patents teach methods of calibrating single spectrometers or transferring spectral information among spectrometers requiring the intervention of a skilled analyst or the use of training or representative samples. The above methods are generally not suitable for transferring chemometric models in an industrial environment or setting and all would create adversity to the successful election, selection, and implementation of spectroscopy equipment in such an environment. In order to realize the potential benefits of spectrophotometrics in manufacturing facilities, chemical plants, petroleum refineries, and the like, a method is required whereby the impediments to accurate transfer of chemometric correlations are identified, quantified, and solved.
It is therefore an object of the present invention to provide a method of transferring spectral information among spectrometers that achieves superior transfer accuracy.
It is an object of the present invention to provide a method of transferring spectral information among spectrometers that does not require the use of training or representative samples.
It is another object of the present invention to provide a method of transferring spectral information among spectrometers that is simple and convenient and can be performed by a semi-skilled technician, a computer, and a procedural guideline.
It is another object of the present invention to provide a method of transferring spectral information among spectrometers that can be performed without intervening decision-making steps by a skilled analyst.
It is yet another object of the present invention to provide a process for determining the physical properties of samples utilizing spectrometers utilizing spectral information transferred from another spectrometer.
Other objects appear herein.